This invention relates to a fluidic oxygen sensor which is used to monitor the oxygen concentration of a sample gas.
The oxygen concentration of a sample gas can be determined by flowing a reference gas and the sample gas across a fluidic bridge and measuring the pressure differential in the two channels of the bridge caused by the two gases. Such a device is disclosed in U.S. Pat. No. 4,008,601 issued to Woods.
As further described in the Woods patent, the oxygen concentration of a sample gas can be monitored with a fluidic bridge consisting of two nonlinear orifice resistors and two linear capillary resistors. Sample gas containing oxygen flows through one channel of the bridge consisting of a capillary and orifice, and a reference gas flows through the second channel consisting of a similar capillary and orifice. If the two gases are at identical pressures at the inlet and at the outlet of the bridge, the pressure differential between the two channels at the midpoint between the capillaries and the orifices is a function of the difference in the viscosity and density of the sample and reference gases. If the pressure drop across the entire bridge is proportional to ambient pressure, the bridge differential pressure will be a function of the partial pressure of oxygen in the sample gas.
One of the major problems encountered in the use of a fluidic bridge of the prior art is assuring that the pressures of the sample and reference gases at the inlet and outlet of the bridge are identical. The customary method of providing identical inlet pressures as shown in the Woods patent is to vent the inlet to ambient pressure. Identical outlet pressures are assured by ejecting the sample and reference gases from the bridge with a jet pump. Since the bridge pressure drop must be a function of ambient pressure in order to sense the partial pressure of oxygen, the supply pressure to the jet pump must be controlled by a schedule pressure regulator to provide the desired pressure curve.
The prior art fluidic oxygen sensor as described above has several disadvantages. Since both the capillary and orifice resistors are fixed, calibration of the system is accomplished by adjustable needle valves in the inlet or outlet of each channel. These valves introduce degradation factors which reduce the sensitivity of the sensor. The jet pump which is used to eject the gases from the bridge consumes a supply of forced air. Pressure drop across the bridge caused by the jet pump is difficult to control and is subject to variations caused by altitude and temperature changes. Since the inlet pressure is limited to available ambient pressure, at high altitudes it is difficult to obtain a linear signal readout because of the limited pressure differential. Finally, it is difficult to filter and sample the reference gases at the inlet of the bridge without disturbing the ambient pressure level since after a period of operation the filter in each line begins to clog which increases the pressure drop across each filter to an unknown value.